In the memory shown to Harry by Riddle's diary, we see Hagrid caring for a large, hairy creature in the dungeons. Harry guesses that the hairy creature is the monster in the Chamber, and that Hagrid had been expelled for releasing it. Ron and Harry are unable to query Hagrid about this, however, as he is arrested shortly after they reach his hut. As he is leaving, he tells the boys that they should "follow the spiders."

The spiders lead Ron and Harry into the Forbidden Forest, where they are captured by Aragog's children and taken to Aragog. He says that he was never the monster in the Chamber, and was never near the bathroom where the girl was killed by the monster. He refuses to name the monster, apparently it is so feared by spiders that they cannot even name it. Finally, he says that he cannot stop his children from eating the "friends of Hagrid". Harry and Ron are carried to safety by Mr. Weasley'sflying car, largely feral and still wandering the Forest.

At the beginning of the book we learn from Hagrid that Aragog is very ill. Towards the end he dies and his funeral is a crucial event in the book.

Driven by his greed (Acromantula venom is very expensive), Professor Slughorn allows himself to converse with Harry for an extended period in the presence of mead. Thus Harry is able to convince Slughorn to surrender his memory of Tom Marvolo Riddle, which is crucial to an understanding of Lord Voldemort's plans.

Aragog is mentioned in passing during the last Battle at Hogwarts when one of his descendants bursts through a hole in the castle wall. A large number of his descendants are also mentioned as apparently carrying off Hagrid, and Lord Voldemort is mentioned as having set up camp in the Forbidden Forest where Aragog and his family had once been.

Aragog seems to have a tendency to eat everything around; this may not be a weakness, but is bound to affect his relationships with non-spiders. He is afraid of certain creatures and unwilling to even name them.

Aragog reveres Hagrid as the one who reared him, found him a home and a wife, and protected him in his youth from those like Riddle who would kill him. He controls his brood basically through force of personality, but his control is fading as he ages. While he is willing to speak with "friends of Hagrid", he does not believe he has the ability to protect them from his offspring.

Intermediate warning: Details follow which you may not wish to read at your current level.

In Harry Potter and the Chamber of Secrets we learn that Voldemort, or as he was then known Tom Riddle, had framed Hagrid and Aragog for a murder they did not commit. In Harry Potter and the Half-Blood Prince, through Aragog's death and Hagrid's grief, Harry obtained a piece of information that is vital for the fight against Voldemort. This would seem to be another example of J.K. Rowling’s love of symmetry.

Aragog's being unwilling to name the Basilisk might have been a clue to its nature, but this proves unnecessary; Hermione had already determined the nature of the monster from the other clues, and it is only her being Petrified that has prevented her from revealing this to us. What is revealed is that the monster had killed a girl in a bathroom, and Harry puts these pieces together to come up with Moaning Myrtle, and the possibility that the entrance to the Chamber is in her bathroom.